1. Field of the Invention
The present invention relates to dispensers of two-part compositions having incorporated therein mixing means wherein the flow path of the combined two-part composition within and through the mixing means is readily removable and replaceable.
2. Description of Related Art
Reactive two-part compositions have many applications ranging from general molding compositions to specialized applications such as adhesives, sealants, coatings and potting compounds. Their uses vary from high and low tech industrial production and assembly operations to relatively low-tech consumer applications. They find particular use in OEM manufacture as well in the service sector for general maintenance and repair.
Reactive two-part compositions are of many different formulations and chemistries and include true thermosetting polymers and other curable or settable compositions as well as reactive compositions that show marked increase in viscosity, but do not necessarily solidify. They are most generally characterized as curing/reacting, i.e., undergoing a chemical reaction of components, upon intimate contact of one component of the curable composition with another. Exemplary reactive two-part compositions include those based on polyester, epoxy, acrylic or urethane chemistry. Besides the physical properties and performance characteristics of the cured materials, one of the critical aspects of reactive two-part compositions is their cure speed. All of these aspects play a key role in where and how these materials are used. While physical properties and performance characteristics are, to a large extent, dictated by the chemistry of the polymer or polymerizable component, other additives, if present, such as catalysts, accelerators, and initiators, and their level of incorporation, also affect cure speed, which, in turn, can affect physical properties and performance characteristics. Another important, and oftentimes critical, factor regarding the use of two- or more-part systems is the proper apportionment of the parts as they are being combined. Generally, while these systems offer excellent engineering properties and performance characteristics, their very nature, notably their viscosity and especially their cure speed, can lead to difficulties in mixing, uniformity of cure and properties, and application processes.
For a reactive two-part system of a given chemistry and formulation, optimal performance and cure speed are oftentimes contingent upon careful and intimate mixing of the two parts. Even though polymerization and/or cross-linking of certain two-part curable systems may be self-propagating, i.e., once kicked off the polymerization or cure will continue through the volume of the curable composition, intimate mixing of the component will provide optimal cure speed and avoid stress problems that may arise as a result of too fast a cure in one section of the volume as opposed to another. More importantly, though, those compositions that rely upon the co-polymerization and/or cross-linking of two or more monomers require a substantially homogeneous mixture of the co-monomers in order to ensure good overall cure and performance. The failure to provide such an intimate distribution of the copolymerizable components will result in areas of uncured monomer or co-monomer or of areas having lower than expected, or needed, cross-linking. Similarly, in those curable systems reliant upon curatives including catalysts, cure accelerators, cure initiators and the like, especially those wherein propagation of cure is limited, non-existent or too slow, an even distribution of such curatives in the monomer(s) is important for uniform cure, good cure speed and good performance properties of the cured material. Otherwise one may find areas of poorly cured or uncured materials as well as stresses within the cured materials, especially at the interface of the two regions.
Since cure, and hence viscosity buildup, begins once the necessary reactive components are brought into contact with one another, cure speed then becomes of key concern. At some point in time the viscosity of the formulated composition increases to such an extent and/or the degree of cure that has been attained is so high that the material is no longer capable of being applied or used for its intended purpose. The period from when the materials are first mixed to that point where they are essentially no longer suitable for use is typically defined as its pot life. The slower the cure speed of the system, the longer the pot life and the more time one has to ensure good intimate mixing as well as to apply the activated system to its intended end use. However, a slow reacting system will also take longer to reach the desired cured properties. This may be acceptable for some applications, but for many industrial applications, especially automated bonding, sealing, winding (e.g., filament winding) or potting applications, rapid cure is desirable to speed assembly and improve productivity. Conversely, too fast of a cure speed and little time is available to ensure good mixing, let alone a sufficiently manageable pot life to allow for adequate working of the treated substrates. In some cases, the pot life of these systems may be a minute or two, perhaps even fractions of a minute. While manual mixing and application of these materials may be suitable for a consumer or commercial repair service, it is totally impractical in a production situation—there just isn't enough time.
Thus, industrial and commercial use of reactive two-part compositions requires a delicate balancing of a number of parameters, not just formulation-wise, but also methodology and equipment-wise. The latter, e.g., two-part dispensers and/or mixers, has played a key role in the advancement of the use of such compositions in industrial and commercial applications. Similarly, the importance and versatility of use of these reactive two-part compositions for industrial manufacture and repair/servicing applications has led to many developments in suitable dispensing and mixing equipment. For example, the effective use of these materials has required the development of so called “meter-mix-dispense” systems that automate and control the measuring of the two components, their mixing and application to the part. The entire process can be accomplished within a few seconds.
While these systems have proven effective, some difficulties remain. Although the concern with pot life limitation is greatly diminished by the overall quickness of the mixing and dispensing of the newer apparatus, it doesn't fully go away. Specifically, even though these devices successfully expel most of the curable composition, a thin layer tends to cling to the internal components of the mixer or reside in areas of low or no flow. Because the material is reactive, it will cure in place. As time passes, the layer of cured material builds up and begins to thicken and restrict the flow of uncured material through the dispenser and mixer apparatus. This problem is made worse by the periodic stoppage of the operation of the dispenser and mixer during the daily production cycle, for example, as may occur during breaks, line alterations, line disruptions, line maintenance, etc. Here, since newly mixed material is not there to essentially sweep the wetted surface of the older material, buildup appears to increase even faster. Eventually, the adverse impact on the flow and throughput of the dispenser will necessitate a complete shutdown of the production line while the dispenser is disassembled and cleaned before the line is restarted. In extreme circumstances, the mixture may cure through the volume in the dispenser rendering it useless: thereby necessitating a more difficult cleaning or, worse, outright disposal of the dispenser.
Although most dispensers can be cleaned for re-use, cleaning is labor intensive and usually requires solvents: the latter raising a number of environmental, health and safety concerns. Thus, it may be expedient to simply discard the used mixers on a regular basis and start over with a new one. In this light, single use packaging having distinct compartments containing pre-metered amounts of the components wherein a manipulation of the packaging allows for an intimate mixing of the two components as well as the dispensing thereof helps overcome some of these difficulties while rendering their use nearly fool-proof. However, such single use packaging is all but impractical for high volume, high speed assembly or manufacturing operations, especially automated operations.
One type of mixer often used with two-part compositions is referred to a “static mixer tube” or “Kenics” mixer—after their original manufacturer Kenics Corporation (McCray, U.S. Pat. No. 2,125,245). The tube contains a series of helical elements that divide the stream of material in two and reorient it by 90 degrees before it enters the next element. With each additional element, the number of times the stream is divided and remixed goes up by a power of two. Studies have shown good mixing for this design provided that a large enough number of elements are used. The original static mixers were fabricated from metal and designed for use in the chemical processing industry. However, the design lends itself to low cost fabrication by plastic injection molding where the molded plastic mixers are used once and then discarded.
While popular for many applications, static mixer tubes have a number of limitations. One limitation is that while higher numbers of elements provide superior mixing performance, they also increase the cost of the mixer and increase the flow resistance within the dispenser/mixer apparatus. The many small elements in the tube also make the static mixer tube prone to clogging once any of the material inside begins to thicken. Although plastic molded static mixer tubes are low in purchased cost, the additional costs of labor and lost production time in constantly changing them out must be considered as these costs have a significant impact on the overall process cost.
A second type of mixer suitable for use in a meter-mix-dispense process is the dynamic mixer. These devices are characterized by the use of a mechanically driven element, such as a rotor or impeller blade, an auger, etc., to directly and actively mix the components. There are a number of designs available for use with a wide range of two-part compositions. Specific selection is dependent upon the materials, the application or dispensing process as well as the product viscosities to be encountered by the apparatus.
While extremely effective in mixing performance, the surfaces of the moving elements of the dynamic mixer in contact with the reactive components become covered with hardened material and eventually the device must be disassembled for cleaning. Unlike static mixer tubes, dynamic mixers are costly to manufacture and cannot be economically discarded after use. Furthermore, they are more suited for batch-type processing applications as opposed to continuous processing, especially as may be desired or needed for industrial manufacturing applications, particularly automated operations.
Thus, there remains a need for an apparatus for mixing and dispensing reactive two-part curable compositions which operates in a continuous fashion and in which concern of cure on the surface of the reaction chamber and associated components of the dispenser and mixer device is minimized.
Further, there remains a need for an apparatus for mixing and dispensing reactive two-part curable compositions wherein deposits or buildup of cured material within the apparatus can be removed and/or cleaned with minimal disruption on the operation of the dispenser apparatus.